Rainfall infiltration can affect the stability of the city wall, eventually leading to instability and collapse. This study focuses on the city wall as the object of study and investigate the physical and mechanical properties as well as unsaturated properties of the soil through experiments. Based on the experimental data, a shear strength formula with variable water content is derived, and the safety factor expression is obtained using the Bishop method. Finite element analysis and stability evaluation are conducted on the seepage field of the city wall slope under different working conditions, and the stability change of the city wall and the distribution of pore water pressure during the plum rain season are analyzed. The results show that with the continuous progress of rainfall, the pore water pressure gradually increases, the surface soil begins to saturate, the depth of infiltration continues to increase in the early stage, while the groundwater level rises in the later stage. The stability of the city wall continues to deteriorate during the rainy season, greatly affected by the total amount and duration of rainfall. These findings serve as a theoretical reference for the design and construction of the city wall repair and other projects in the region under typical regional climatic conditions.